The present invention relates to thin film electroluminescent displays.
The configuration of a thin film electroluminescent display according to the prior art is generally shown in FIG. 1. An electroluminescent phosphor 10 is sandwiched between two dielectric layers 12. A transparent conductor 14 and a back electrode 16 are used to address the display.
A pre-eminent difficulty in the commercial application of thin film electroluminescent displays has been the high voltages which are required. Since electroluminescence is produced in the phosphor 10 only at an electric field strength of about a megavolt per centimeter or more, the drive voltages which must be applied are quite high. Typically, the drive voltages required in a thin film electroluminescent display according to the prior art will be substantially higher than those used in plasma display panels, and may be as high as 200 volts or more. Such high display address voltages mean that the display driver circuits required are very expensive and physically large. However, since the dielectric layers 12 have a finite dielectric constant, not all of the AC voltage which is applied to the conductors 14 and 16 appears across the phosphor 10. Thus, significant portions of the large applied voltages are not strictly required to achieve the desired electroluminescence.
A further possible source of voltage inefficiency is caused by the finite resistivity of the dielectric layers 12. The ohmic current which flows vertically in the dielectric layers 12 imposes a further burden on the display drivers, and, if the dielectric loss of the dielectric layers 12 is high enough, further voltage inefficiency is caused.
The joint constraint of these two problems is difficult to satisfy. For example, if titanium dioxide were used for the dielectric, its very high dielectric constant (above 100) would mean that nearly all the applied voltage would appear across the phosphor. However, it is difficult to deposit titanium dioxide films with a resistivity of much more than 108 ohm cm, and such a low resistivity would cause dielectric loss which would more than offset the advantage obtained through the high dielectric constant of titanium dioxide. Conversely, dielectrics such as alumina or silicon nitride have adequately high resistivity, but have relatively low dielectric constants (less than 10), so that much of the applied voltage appears across the dielectric layers 12 rather than across the phosphor 10.
Ferroelectric materials such as barium or lead titanate have high dielectric constants. However, it is very difficult to deposit thin films of these materials by evaporation as the melting points of the component oxides (i.e., in the case of barium titanate, barium oxide, and titanium dioxide) are widely different. In order to achieve the perovskite phase, which is essential for achieving high dielectric constant, the substrate temperature must be higher than 550 degrees C. Thus the process can not be used on ordinary glass. Moreover these materials have high loss and low dielectric strength.
Thus, it is an object of the present invention to provide a material for the dielectric layers of thin film electroluminescent display devices which has both a very high resistivity and a very high dielectric constant.
It is a further object of the present invention to provide electroluminescent display devices requiring only a minimal applied voltage.
It is a further object of the present invention to provide electroluminescent display devices having a large ohmic resistance and a large capacitance in series with the active phosphor of each pixel.
It is a further object of the present invention to provide a method for fabricating thin film electroluminescent displays having good voltage efficiency.
Further references regarding TFEL displays, all of which are hereby incorporated by reference, include Electroluminescence (ed. J. Pankove, 1977); Hurd & King, Physical and Electrical Characterization of Co-Deposited ZnS:Mn Electroluminescent Thin Film Structures, 8 J. Electronic Materials 879 (1979); Tanaka et al, Evidence for the Direct Impact Excitation of Mn Centers in Electroluminescent ZnS:Mn Films, 47 J. Applied Physics 5391 (1976); Krupka, Hot-Electron Impact Excitation of Tb+++Luminescence in ZnS:Tb+++Thin Films, 43 J. Applied Physics 476 (1972).
Titanium dioxide has an excessive dielectric loss. Some typical values for the dissipation factor (which measures dielectric lossiness) are for alumina, 0.001 or less; for titanium dioxide, around 0.01, for ZnS, around 0.005. (Since these values are heavily dependent on process parameters, they should be regarded only as rough indicators.) If the dielectric has a dissipation factor much greater than that of the phosphor, substantial thermal dissipation in the dielectric will result. Thus, a desired objection is a dissipation factor of 0.005 or less, with a dielectric constant of 30 or better.